1. Field of the Invention
This invention relates generally to frequency-shift keyed data link systems and particularly to method and apparatus for synchronizing said systems and recovering information transmitted when such a system is operated in an interference prone environment.
2. Prior Art
Digital transmission of information makes efficient use of transmission facilities since more information may be packed into a given transmission period than may be done with voice or other analog communication schemes. Digital transmission is in the form of pulses organized in a manner to convey information.
If only one message is to be transmitted across a message carrying medium there is no doubt that the pulses received relate to the message transmitted. However, if two or more messages are to be sent across the same medium simultaneously, some means of identifying a given pulse with a given message must be provided if communication, in the true sense of the word, is to occur. For this reason pulses are often identified with a given message by pulse code modulation (PCM) techniques. Frequency-shift keying (FSK) is one form of PCM.
With FSK, a given message is assigned a corresponding frequency channel to carry the information across the message medium. The message medium may be air, water, wire lines, electromagnetic fields, etc. Within the frequency channel, a pair of frequencies may be assigned; one to indicate the presence of a MARK, the other the presence of a SPACE. The mark/space terminology derives from the early days of telegraphy where the MARK indicated the telegraph key was closed and current flowed.
Information is derived from a particular coded grouping of marks and spaces referred to as a "frame". When a series of marks and spaces are transmitted, it is important that the receiver be able to identify a particular mark or space as belonging to a given encoded group or frame. If the identification process fails, gibberish may result at the receiver output and the receiver is said to be out of synchronization with the transmitter.
Similar nonsensical results may derive if a given MARK or SPACE is obliterated by interference or if the interference causes a MARK or SPACE signal to appear on the receiver line where no or the opposite sense signal was transmitted.
Synchronization and interference become more difficult problems as size of the data link system increases. As signals are transmitted over longer distances across varied media their time scale tends to differ and they become more sensitive to interference.
Prior art synchronization techniques fall into two categories:
The fully synchronous approach where an attempt is made to synchronize and drift-compensate all the clocks used throughout the system. This means all clocks must display the same operating speeds;
The quasi-synchronous approach accepts a close but imperfect clock correlation which means the message will be distorted. To overcome the effects of such distortion, pulse stuffing techniques may be resorted to: Dummy characters are added to the transmission and the distortion is confined to these dummies so far as is possible.
Some systems make use of synchronizing pulses added to each encoded information frame. When additional channels are added to the system it is advantageous to add synchronizing pulses to synchronize each channel rather than trying to synchronize groups of channels. As the information content, speed of data transmission, and number of channels increase, the need for such additional synchronizing pulses places an added burden on the system.
It is an object of the present invention to provide method and apparatus for synchronizing a data link system without the need for correlating multiple system clocks or for providing additional synchronization pulses within each information frame (group) or within each channel.
Interference can destroy transmitted synchronization signals as well as gobble the information content of a message.
It is a further object of the invention to provide method and apparatus to enhance system ability to remain synchronized in the presence of interference.
It is the inventor's intention to also supply method and apparatus for determining the data state (i.e., MARK or SPACE) of the signal transmitted even though the received signal has been degraded by interference signals approaching the received signal in amplitude and frequency. To this end a digital frequency detector is disclosed which functions to demodulate and recover the digital data from the received mark/space signals, even in the presence of an interfering signal of the same frequency, and at an amplitude approaching that of, the true signal.